solve validate binary search tree

Author: delight010

validate-binary-search-tree/delight010.swift

@@ -0,0 +1,19 @@
+class Solution {
+    private var prev: Int?
+    
+    func isValidBST(_ root: TreeNode?) -> Bool {
+        inorder(root)
+    }
+    
+    private func inorder(_ root: TreeNode?) -> Bool {
+        guard let root = root else { return true }
+        
+        guard inorder(root.left) else { return false }
+        
+        if let prev = prev, root.val <= prev { return false }
+        prev = root.val
+        
+        return inorder(root.right)
+    }
+}
+